modules: use NGINX_FROM_IMAGE to specify which image to start with.

modules/Dockerfile

-ARG NGINX_VERSION_FROM=mainline
-FROM nginx:${NGINX_VERSION_FROM} as builder
+ARG NGINX_FROM_IMAGE=nginx:mainline
+FROM ${NGINX_FROM_IMAGE} as builder
 
 ARG ENABLED_MODULES
 
@@ -69,7 +69,7 @@ RUN set -ex \
     done \
     && echo "BUILT_MODULES=\"$BUILT_MODULES\"" > /tmp/packages/modules.env
 
-FROM nginx:${NGINX_VERSION_FROM}
+FROM ${NGINX_FROM_IMAGE}
 COPY --from=builder /tmp/packages /tmp/packages
 RUN set -ex \
     && apt update \

modules/Dockerfile.alpine

-ARG NGINX_VERSION_FROM=mainline-alpine
-FROM nginx:${NGINX_VERSION_FROM}-alpine as builder
+ARG NGINX_FROM_IMAGE=nginx:mainline-alpine
+FROM ${NGINX_FROM_IMAGE} as builder
 
 ARG ENABLED_MODULES
 
@@ -61,7 +61,7 @@ RUN set -ex \
     done \
     && echo "BUILT_MODULES=\"$BUILT_MODULES\"" > /tmp/packages/modules.env
 
-FROM nginx:${NGINX_VERSION_FROM}-alpine
+FROM ${NGINX_FROM_IMAGE}
 COPY --from=builder /tmp/packages /tmp/packages
 RUN set -ex \
     && . /tmp/packages/modules.env \